Method of improving flow of stock from the stock inlet of a paper machine



BOONE ETAL 2,832,268 METHOD OF IMPROVING FLOW OF. STOCK FROM THE STOCKINLET OF A PAPER MACHINE Filed July 29, 1954 April 29, 1958 5Sheets-Sheet 1 Ig.E

April 29, 1958 G. c. BOONE ETAL 2,832,268

METHOD OF IMPROVING FLOW OF STOCK FROM THE STOCK INLET OF A PAPERMACHINE Filed July 29, 1954 3 Sheets-Sheet 2 Apnl 29, 1958 G. c. BOONEET AL 2,832,268 7 METHOD OF IMPROVING FLOW OF STOCK FROM THE STOCK INLETOF A PAPER MACHINE Filed July 29, 1954 3 Sheets-Sheet 3 States UniteMETHOD or nurnovnvo snow on STOCK FROM THE s'rocu INLET or A PAPERMACHINE Application July 29, 1954, erial No. 446,566. 9 Claims. (c1.92-44 This invention relates to paper making and more particularly to amethod of controlling the condition of the stock as it is introducedupon the forming table of a Fourdrinier paper machine. I g I Theinvention consists essentially in the provision of new and improveddevices forming part of the stock inlet of a paper machine ashereinafter more particularly described. These devices and the effectswhich they produce are illustrated in the accompanying drawings inwhich:

Figure 1 illustrates the phenomenon of separation of How occurring at asurface along which there is an 'adverse pressure gradient;

Figure 2 illustrates the rol ing up of two thin vortex layers behind acylinder disposed transversely, in a stream of viscous liquid at lowvalues of the Reynolds nuinber;

Figure 3 shows a stage in the transition of thehow pattern as thevelocity of the stream of viscous liquid past the cylinder is increased;i

Figure 4 shows the further stage in the transition of the flow patternwith increasing velocity at which vortics break away into the mainstream of the liquid;

' Figure 5 illustrates the final stage in the transition with increasingvelocity showing the Karman vortex street which is formed;

Figure 6 shows the relation between the Reynolds numberand the frequencyof discharge of the vortices;

Figure 7 shows a Karman vortex street confined within a convergingchannel;

igure 8 illustrates means for varying the position of the flow-modifyingmember; p i V Figure 9 illustrates a preferred embodiment of our ven o yN l Figure 10 illustrates a variant of a form of nonstreamlinedflow-modifying member; and Fig ure ll illustrates a substantiallystreamlined flowmodifying member. l v In paper making, thefunctions of astock inlet are to receive the stock and project the stock onto themoving wire cloth in a suitable state of agitation, with uniform volumeper unit time and with uniform consistency, direction of flow andvelocity. In conventional practice, stock inlets include various devicesdesigned to promote one or more of the above functions. A particulardevice may, however, be favourable to one function and unfa curable toanother so that in the aggregate, the conction of stock inlets incommercial use includes provisions which effect a compromise betweenvarious ideal requirements. Thus, for example, the newer stock to thepaperinachine maybe spread. from a single pipe across the full width ofthe machine by first dividing the flow into two streams which are theirfed into the stock inlet from opposite sides. This arrangement providesfairly uniform distributional the stock across the machine butintroduces undesirable crossflow eddies. In

sadism t su n r th s si blstssm ere forated rolls or similar devices maybe interposed in the 'ice stock inlet at a point between the stock entrypoint and the slice. These are, however, only partially effective and,if placed too close to the slice, cause the how of stock from the sliceto divide into a number of separate streams. A further method is toemploy a series of flat vertical plates or fiow-eveners close to theslice in order to reduce cross-eddies in the stock how. The introductionof such flow-eveners tends, however, to divide ti e flow into a seriesof separate streams across the Width of the machine. A device which isthus introduced to reduce cross-eddies in the flow, therefore, itselfpromotes an undesirable systematic variation in the condition of thestock as this is discharged across the width of the paper machine. Inexisting machines, these various de vices have been used or combined ina measure of compromise to provide a relationship tending in theaggregate toward a preferred type of flow at the point of dis charge. Asthe speed of a machine is increased, such comprises become lesssatisfactory and, in many cases in practice the maximum operating speedof a machine is restricted by the inability of the stock inlet toprovide a suitable flow of stock onto the wire.

In particular, the flow onto the wire may have thick and thin streakswhich are caused by the nature of the flow to the slice and which cannotbe. corrected by adjustments in the opening of the slice. Further, someof the flow-eveners and other devices preceding the slice in the line offlow, as hereinbefore mentioned, produce disturbances which persistthrough the slice onto the wire. Finally, the condition of the stock asit flows onto the wire changes as the speed of the machine is increased.Thus, for example, the fibres become more highly orientated parallel tothe machine direction and it becomes more difficult to make the desiredquality of paper.

We have discovered that these limitations of stock in let design may beovercome and the machine speed increased by several hundred feet perminute. In order to illustrate our invention, two particular embodimentsof means will be described, but it will be: realized that it is possibleto employ other means to achieve the same result without-deviating fromthe spirit of the invention as herein described and claimed.

It is Well-known in fluid dynamics, that when a viscous liquid flowsalong a fixed boundary there is a thin layer, called the boundary layer,in contact with theboundary in which the viscous stresses areappreciable without being predominant. The liquid in immediate contactwith the boundary is theoretically at rest. The motion of the liquid inthe boundary layer is governed by three factors. It is retarded byfriction at the boundary Wall, it is pulled forward by the rest of theliquid through the action of viscosity, and it is retarded oraccelerated accordingly as the pressure gradient along the surface isadverse or fa vourable. If the gradient is favourable it continues itsforward motion and the thickness of the layer remains small. If thepressure gradient is adverse, however, the retarding forces predominateand the boundary fiow is first halted and then the adverse gradient maycause flow in the reverse direction. This reverse flow causes theboundary layer to widen and the main how to separate from the boundary.These phenomena are illus trated diagrammatically in Figure 1 in whichthe char acteristics of motion of thin strata of fluid are indicated.The fluid flowing from left to right in Figure l with velocity Uencounters an adverse pressure gradient. At the point P the forwardstream leaves the surface and a back flow in the direction of thepressure gradient occurs. Thechain-dotted line B L indicates the limitofthe boundary layer and the chain-dotted line P R the limit of theregion of back flow. Broken lines in the figure marked by arrow headsindicate the course of stream-lines. The point of separation is thepoint at which the boundary flow reverses. With viscous liquids the flowof the liquid in a boundary layer gives rise to vorticity. If noseparation of flow occurs at the edges of the boundary layer then thevorticity is confined to the boundary layer proper. When separationoccurs, however, the vorticity is shed into the main stream. If noseparation of flow occurs from the surface of a body then the body issaid to be streamlined. In practice the term streamlined is extended toinclude those bodies for which separation of flow at the surface, whenit does occur, does so very near to the downstream extremity so. thatthe fluid closes in again behind the body producing only a very narrowwake. When we use the word streamlined, we refer to the extendeddefinition as above.

This invention describes a method whereby the phenomenon of separationand resulting diflusion of vorticity into the main stream may beadvantageously utilized to appreciably assist in overcoming many of thepreviously discussed undesirable features in the flow of stock onto thewire. The conditioning of the'stock produced by our invention arisesfrom a rapid oscillatory movement between adjacent regions of the streamof stock. The amplitude of these movements affects the quality offormation in the sheet of paper produced. For a given amount of energyintroduced into the flow the formation improves as the amplitude of themovement is reduced. A second purpose of this invention is to ensurethat the vortex system is of high frequency and that the energy requiredin the stream to condition the stock produces relative motion of smallamplitude. Our invention may first be illustrated by reference to oneembodiment of the means employed by us in the operation of theinvention.

When a viscous fluid flows past a circular cylinder, two symmetricallines of separation occur on the downstream side. From them two thinvortex layers leave the cylinder and roll round on themselves, thevorticity becoming more pronounced in the rolled-up portion, asillustrated in Figure 2. If the velocity of the liquid is graduallyincreased the size of the vortices increases and they move further awayfrom the cylinder, as illustrated in Figure 3. Eventually a stage isreached at which the system becomes unstable, the vortices break awayinto the main stream, and the formation of other vortices then takesplace, as illustrated in Figure 4. In the case of a circular cylinderthe vortices are successively discharged from alternate sides and form adouble layer of vortices in the wake of the cylinder in which thevortices from one separation line are spaced between the vortices fromthe other line. Such an arrangement is technically known as a Karin-anvortex street and certain of its properties are understood. Such asystem is illustrated in Figure 5.

Such knowledge as previously existed relative to a Karman vortex streetwas largely restricted to its behaviour in a liquid in which boundarysurfaces other than that of the cylinder are so far removed as to haveonly a negligible eflect on the flow adjacent to the cylinder. In such acase the frequency N at which the vortices are discharged, the diameterd of the cylinder, the velocity U of the stream, the viscosity a and thedensity of the fluid are related and the experimental results are bestsummarized graphically by the inter-dependence of two dimensionlessfunctions, namely, the Reynolds numbe Re that is Ud Nd T and Thisinter-dependence has been discussed in Modern Developments in FluidDynamics, edited by S. Goldstein,

Oxford University Press, 1950, vol. 2, sections 184 and 247, and isillustrated in Figure 6. According to this reference for values of Rebetween 10 and, 10 the discharge of vortices is periodic; above thatrange the discharge is aperiodic and there is a sudden increase in theaverage frequency.

In the course of our investigations we have applied this knowledge tothe paper making art and have discovered that, when a Karman vortexstreet is confined within a converging channel such as exists in a stockinlet, and as illustrated in Figure 7, certain features of the behaviourof vortex streets may be so regulated and controlled as to be ofadvantage in the production of paper on high speed machines. We havefound that the same form of inter-dependence between the two quantitiesRe and hi U

occurs with stock flowing past a cylinder in a converging channel. Underthe high velocity gradients, in a stock inlet slice the viscosity ofstock has been found to 'be fairly close to that of water at the sametemperature and for the purposes of our invention the viscosity of watermay be used in the calculations. We have discovered that in a stockinlet slice some modifications are introduced because other boundarywalls are no longer remote from the cylinder. The value of U now becomesthe velocity which would have occured at the position of the cylinderhad the latter not been present. We have found that, while the form ofthe curve of interdependence is similar to that shown in Figure 6 thecurve is displaced horizontally to an extent which depends on thegeometry of the walls of the slice and the cylinder. We found, forexample, that in the case of a stock inlet slice the displacement wassuch that the transition from periodic to aperiodic discharge ofvortices occurred at a value of Re of the general order of 5 x10 and thetransition point moved slightly towards higher values of Re onincreasing the diameter of the cylinder.

A system of vortices is only stable under special conditions and it isan important feature of this invention that the conditions be chosen togive, an instability in the system of a specific nature. It will beobvious to those skilled in the art that unless the conditions describedin this invention are carefully observed the resulting effect on theflow of stock onto the wire, rather than permitting an increase in themachine speed, may cause it to be seriously retarded.

Instability may arise from several causes although two are especiallyimportant in the operation of this invention. First, if the velocity ofthe liquid is not uniform and is greatest at the position of thecylinder in the stock inlet slice, then a two-dimensional instabilityarises. Secondly a form of instability arises from disturbances alongthe length of the cylinder. Such a disturbance could arise fromnon-uniform flow in the slice. The second form of instability causes adistortion of the vortex pattern, in which the vorticity spreads intothe system ultimately destroying the pattern. It is an important featureof this invention that the conditions for creating the particular typeof instability required in the system be so chosen by selection ofsuitable location and dimensions of the cyl' inder that the spreading ofthe vorticity is almost entirely carried out under control, while thestream is still confined within the walls of the stock inlet slice.Unless this is done the issuing stream of stock bursts into a spray atthe end of the slice. Such an issuing stream would require a largeregion of the wire with negligible drainage so that it might reform,otherwise the paper would have a very patchy formation and trouble wouldbe experienced from air bubbles trapped in the stock.

The choice of the cylinder dimensions and locationis governed by severalcriteria:

( 1) It is desirable, in order to obtain the maximum advantage from theinvention, to work with aperiodic discharge of vortices, i. e., with Resin excess of 5X10. We have found that Reynolds numbers as low as 10 maybe efiective but we prefer that Reynolds number he F in excess of x10 asabove. With existing machines in which the profile of the stock inletslice approach is predetermined the Re for a cylinder at any positioncan be calculated in terms of the viscosity of the stock and itsvelocity at the stock outlet.

(2) The frequency of discharge of the vortices should be as high aspossible. This is attained by either working in the aperiodic zone asabove or by using a small diameter cylinder or by a combination of both.

(3) The cylinder must be located at a distance back of the slice outletsufiicient to allow the vortex street to expand to the entire thicknessof the stream before discharge. If the vortex street remains merely inthe central portion of the stream the expansion of the vortex systemcontinues outside the slice at such a rate as to cause the stream tobreak up, which is undesirable.

(4) The diameter of the cylinder must not be so small as to cause fibersto cling onto it and so start lumps which will subsequently break away.The minimum diameter will vary with the nature of the furnish of thestock. In our work we have found that rods in excess of one half inchdiameter will stay clean in a newsprint furnish.

(5) When the flow across a section of the stock inlet back of the sliceis not uniform, i. e., the value of U varies, then the system ofvortices obtained is sensitive to the position of the cylinder relativeto the two walls of the stock inlet.

(6) It is undesirable that the elements supporting the cylinder extendsubstantially beyond'the cylinder towards the slice.

In the application of our invention to paper machines where it isdesirable to operate over a range of operating speeds and where suchrange is suflicient to introduce changes in the flow distribution in theregion of the slice, our invention may be more readily applied if adevice is provided which permits adjustment of the position of theflow-modifying member to maintain the desired conditions of flow asherein set forth under various conditions of machine speed. An exampleof how this may be accomplished is shown in Figure 8 in which 1 is theslice lip held in position by adjusting rod 2 with relation to apronplate 3, 4 indicates one of those flow-evener plates which have beenextended to support the flow-modifying member 5. The extendediloW-evener plates such as 4 are disposed on hinged shaft 6 and shaft 7mounted in eccentric bearing 8 which latter eccentric bearing permitsadjustment of the extended flow-evener plates and thus of the relativeposition of the flow-modifying member S.

In a second or preferred embodiment of our invention,

we have found that the development of two systems of vortices fromflow-modifying members disposed on the slice lip and apron plateproduces a stream of stock with less tendency to expand on projectionfrom the slice onto the wire, the vortices discharged are alwaysaperiodic, and the frequency of discharge is much greater than when thepreviously described embodiment is employed under similar conditions.

When a stream leaves the outlet of a converging channel it continues tocontract in width for a short distance depending on the approach flow tothe outlet, until the pressure of the stream is equal to that of thesurrounding air and the direction of flow in the stream is uniform. Astable stream in the absence of retarding forces would then continue inthis condition. In the presence of air, however, the stream is graduallyretarded and, in order to maintain continuity, the stream broadens. Therate at which the stream broadens depends, inter alia, on theretardation produced by the air and this in turn varies with thecondition of the surface of the stream. As previously explained, arapidly expanding stream is very unstable and quickly disintegrates intospray. Such a stream entraps large quantities of air and in the case ofa stream of stock flowing from a stock inlet slice, requires a longregion of restricted drainage on the forming 6 wire to reform beforesubstantialdiainage ancersrnence. One of the purposes of this preferred'embbdiment is to ensure that the stream of stock remains compact for aconsiderable distance after leaving the slice.

The preferred embodiment of our invention is illustrated in Figure 9. Inplace of the cylinder constituting the flow-modifier of the firstembodiment of our invention, We dispose in the converging channel aplurality of projections (preferably two), one from each of the apronplate and slice lip, such projections being of non-streamlined surfaceand substantially hemispherical cross section. Constructionally theseprojections may be conveniently obtained by fixing cylinders ofsemi-circular cross section onto the plane surface of the channel.Alternatively one or both of the projections may form part of a hingewhich permits movement of the slice lip or the apron plate, as the casemay be. At the rate of flow encountered in paper machine slicesseparation occurs on the down stream side of both projections andvortices are shed into the body of the flow. v

There are several important differences betweentlie vortex sysem shed bytwo projections and that shed from a single cylinder in the channel, aspreviously described. In the case of a single cylinder the pressuredistribution in the wake of the cylinder causes the vortices to be shedalternately from the two lines of separation to form a Karman vortexstreet. This gives rise to a corrugated flow pattern from the slice andthe resulting stream spreads more than would be the case without acylinder. The improvements in the uniformity and condition of the streamproduced by a cylinder however, outweigh the disadvantage of thereforming zone which is necessary on paper machines operating athighspeed s. At very high speeds, however, the balance becomes adverse.The vertices in the two streams of vortices produced by the ttivoprojections are not alternately spaced, but are shed independently ofone another. They do not combine, there fore, to produce a regularcorrugated flow pattern. Furthermore, they are generated on the outsideof the fiow and diffuse inwardly. These two differences give rise toseveral distinct advantages from a practial papermaking point of view.We have found that if the systin is designed to fulfill certainrequirements, as hereinafter described, the stream is such as willenable stock a condition suitable for paper making to be fed onto theforming wire at much higher speeds than heretofore. One of theseadvantages is that the stream shows less tendency to expand on leavingthe slice than cohditions employing the previouly describedernbodinlentiof our invention and therefore provides less opportunityair entrapment at corresponding speeds. The desired energy forconditioning the flow for good sheet formation can be introduced with aslittle as one half of the amplitude of relative movement betweenadjacent s eg meiits" of the stream as that produced by a singlecylinder in the channel. The respective frequencies ofdischar geo'f thevortices on the two sides of the stream are aperiodic and independent ofone another so that the vortices do combine to produce corrugation ofthe whole stream. Finally, we have found that the frequency range in wliich the discharge of the vortices occurs is approximately twice thatproduced by a single cylinder under the same'fiow conditions.

Several criteria must be satisfied in the design of this preferredembodiment of our invention:

(1) The minimum cross-channel distance between the two projections (orbetween any two of such projections if more than two are employed)should be at least equal to the maximum vertical slice opening in orderto avoid reducing the sensitivity of control exercised by the sliceopening on the flow.

(2) The projections should be at a sufficient distance from slice outletto enable the two vortex streams to diffuse to the centre of the streamof stock, otherwise the energy distribution is too high at the twosurface layers of the stream and causes drops to break away. (3) Thedimension of the projection is determined first by the energy requiredin the stream of stock and this depends on the design of the stock inletas a whole. The

, greater the dimension of the projections, the greater the energyintroduced. The energy introduced is a function of the'velocity ofthe-stream and the dimension of the projections. Secondly, the frequencyrange of the variations is a function of the Reynolds number, thediameter of the projections and the velocity of the stream. Thefrequency should be made as high as possible compatible with the othercriteria.

"It will be obvious that in all embodiments of our invention thedesigned contour of the flow-modifying devices should be substantiallymaintained during continuing use and such devices should therefore beconstructed of materials which will resist corrosion and erosion.

I When we use the term cylinder in this specification we mean athree-dimensional body which has a constant cross section perpendicularto an axis. .When we use the termTcyIindricaI body in the claims we meana threedimensional body which has a constant cross-section perpendicularto an axis and which is non-streamlined as herein defined.

In the embodiments of our invention as hcreinbefore discussed andillustrated we have used as examples of the non-streamlinedflow-modifying member, either a circular cylinder (Figures 2 to 5 and 7)or a semi-circular cylinder (Figure 9). Such simpler forms ofnon-streamlined cylinders are recommended for the convenience of theirconstruction and operation. The flow characteristics consequent upon theuse of such simpler forms are also more readily derived and it will beappreciated that extreme complexities of flow patterns may result fromthe use of more complicated non-streamlined shapes. In practice also itis difi'icult to obtain and maintain a constant cross section -fornon-circular shapes. Where a constant cross section is not maintainednon-uniformities of the jet across the machine arise which are highlyundesirable. For these practical reasons we prefer to use the simplerforms of non-streamlined shapes as herein illustrated. We havenevertheless employed and evaluated other shapes of flow-modifyingmembers and by way ofillustration Figure 10 shows a flow-modifyingmember of pear-shaped cross section which, subject to the limitations ofits construction as noted above, being non-streamlined,

was capable of adequately accomplishing the results of our invention asabove described. In Figure 11, however, is shown a flow-modifying memberthe cross section of'which is substantially streamlined i. e.,streamlined within the jdefinition of this term as used in thisspecification. In practice we found that the use of a member having thisor a similar cross section failed to produce 7 the desired effect.

It is to be understood that the invention is not limited 'to the abovespecifically-described embodiments of the stock inlet onto theFourdrinier wire of a paper machine which comprises disposing in thestock inlet transversely of the flow of stock a stationary cylindricalbody having at least two lines of fiow separation thereon at a positionsuch that the flow of stock past the said cylindrical body ischaracterized by a Reynolds number in excess of 10 whereby a series ofaperiodic vortices is shed from each of said lines into the flow and,prior to projection of stock onto the wire, maintaining the flow in aconfined channel until the dilfusion of vortices is substantiallycomplete.

2. A method of improving the flow of stock from the stock inlet onto theFourdrinier wire of a paper machine which comprises disposing on each ofthe slice lip and apron plate of the stock inlet transversely of theflow of stock at least one stationary cylindrical body each such bodyhaving at least one line of flow separation thereon at a position suchthat the flow of stock past the said cylindrical body is characterizedby a Reynolds number in excess of 10 whereby a series of aperiodicvortices is shed from each of such lines into the flow and, prior toprojection of the stock onto the wire, maintaining the flow in aconfined channel until the diffusion of vortices is substantiallycomplete.

3. A method of improving the flow of stock from the stock inlet of apaper machine according to claim 1 in which the stationary cylindricalbody is a circular cylinder.

4. A method of improving the flow of stock from the stock inlet of apaper machine according to claim 2 in which at least one of thestationary cylindrical bodies is a semi-circular cylinder.

5. A method of improving the fiow of stock from the stock inlet of apaper machine according to claim 1 in which the Reynolds number is inexcess of 5x10.

6. A method of improving the flow of stock from the stock inlet of apaper machine according to claim 2 in which the Reynolds number is inexcess of 5 X10 7. A method of improving the flow of stock from thestock inlet of a paper machine according to claim 1 in which thediameter of the stationary cylindrical body is in excess of one-halfinch.

8. A method of improving the flow of stock from the stock inlet of apaper machine according to claim 1 in which the stationary cylindricalbody is supported by means not substantially extending beyond said bodytoward the slice.

9. A method of improving the flow of stock from the stock inlet of apaper machine according to claim 2 in which the cross-channel distancebetween any two of such stationary cylindrical bodies is not less thanthe maximum vertical height of the slice opening.

References Cited in the file of this patent UNITED STATES PATENTS1,667,755 Valentine May 1, 1928 1,898,372 Hyde Feb. 21, 1933 1,909,150Bell-Irving et a1 May 16, 1933 1,968,028 Clements July 31, 1934 FOREIGNPATENTS 370,422 Germany Apr. 30, 1921 353,133 Great Britain July 23,1931

